The field of art to which this invention pertains is synthetic resins containing a hydrophilic group, said resins being soluble or dispersible in water when salted.
Industrial coating processes utilizing aqueous dispersions or solutions of organic resinous film forming compositions continue to grow in importance. The aqueous coating compositions are used in various applications, such as spray coating, flow coating and electrodeposition coating processes. Particularly useful organic resinous film forming compositions are cationic compositions which, primarily, are used in primer paints for metals. Such cationic compositions which contain amine nitrogen groups have superior corrosion resistance when formulated into primer paints.
The coating of electrically conductive substrates by electrodeposition is an important industrial process. In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous dispersion of film-forming polymer. An electric current is passed between the article and a counterelectrode in electrical contact with the aqueous dispersion until a desired amount of coating is produced on the article. The article to be coated can be made the anode or the cathode depending upon the ionic nature of the coating system.
There are certain disadvantages in anodic deposition processes. Anodic deposition on ferrous metals tends to discolor the electrodeposited film and phosphate conversion coatings, which are commonly applied to a metal surface before an organic coating composition is deposited thereon, tend to be stripped from the metal under anodic deposition conditions. In addition, it is a peculiarity of anodic electrodeposition coating methods that nascent oxygen is produced at the anode which can react with the resinous coating composition to produce bubbles or voids in the deposited coatings.
The use of cathodic electrodepositable compositions tends to alleviate the discoloration problems and to give improved resistance properties. Although nascent hydrogen develops at the cathode during the electrodeposition process, no metal ions pass into the coating solution or are present in the deposited film. Generally, the amount of nascent hydrogen produced at the cathode does not have the same deleterious effect on the properties of the deposited film as does the nascent oxygen produced during anodic deposition.
Cationic coating compositions generally are derived from resinous compositions containing a basic nitrogen atom which can be salted with an acid and then be dissolved or dispersed in water. Sufficient basic nitrogen atoms should be present so that dispersibility or solubility can be obtained with a minimum amount of acid. If the coating bath is very acidic, considerable corrosion problems with the metal tanks, piping systems and other equipment are encountered.